The cytochrome P450 enzyme class includes the enzymes allene oxide synthase (AOS), hydroperoxide lyase (HPL) and divinyl ether synthase (DES). They form a separate subfamily referred to as CYP74.
Owing to the multiplicity of cytochrome P450 enzymes, a nomenclature was developed which assigns a specific family and subfamily to each protein of this class on the basis of its primary structure. Thus, all the AOSs form their own subfamily CYP74A, while CYP74B comprises the 13-HPLs, CYP74C the 9/13-HPLs and CYP74D the 9-DESs (Feussner et al.; 2001, Trends Plant Sci. 6, 268-273). Proteins from the same subfamily are numbered chronologically.
CYP74 enzymes are monooxygenases with a hemamolecule of prosthetic group. Although they also have a protoporphyrin IX group (hem b) attached as prosthetic group, they have very little affinity to CO (Matsui, 1998, Belgian Journal of Botany. 131, 50-62).
They constitute important enzymes in the metabolism of polyene fatty acids, which is known as the lipoxygenase (LOX) pathway (Feussner and Wasternack, 1998, Fett/Lipid. 100, 146-152).
LOXs are dioxygenases in which the iron in the catalytic center is bound to amino acid side chains (Brash, 1999, J. Biol. Chem. 274, 23679-23682). They catalyze the incorporation of molecular oxygen into the (1Z,4Z)-pentadiene system of polyunsaturated fatty acids. In plants, these are mainly linoleic and α-linolenic acid. Depending on the regioselectivity of the LOX employed, two different positional isomers of hydroperoxides, viz. (9S) isomers or (13S) isomers, may arise as products. For example, α-linolenic acid gives rise to (13S,9Z,11E,15Z)-13-hydroperoxy-9,11,15-octadecatrienoic acid (13S-HPOTE) and linoleic acid gives rise to (13S,9Z,11E)-13-hydroperoxy-9,11-octadecadienoic acid (13S-HPODE).
In plants, these hydroperoxides are rapidly reacted further by a multiplicity of enzymes. Currently, seven different enzyme families which convert hydroperoxides and thus compete for LOX products are known in the plant kingdom: the allene oxide synthase (AOS) reaction, the hydroperoxide lyase (HPL) reaction, the divinyl ether synthase (DES) reaction, the reductase reaction, the peroxygenase reaction, the epoxyalcohol synthase (EAS) reaction and the LOX reaction itself (Feussner et al.; 2001, Trends Plant Sci. 6, 268-273). When 13-HPOTE is reacted in the presence of the enzyme allene oxide cyclase (AOC), a cyclization reaction takes place which gives 12-oxophytodienoic acid (12-oxo-PDA) (Ziegler et al.; 2000, J. Biol. Chem. 275, 19132-8), which, in turn, is the precursor of jasmonic acid, which is here considered a plant hormone.
AOS (EC4.2.1.92; CYP74A) was the first CYP74 enzyme to be described; the homogeneous protein was first isolated from flax (Song and Brash, 1991, Science. 253, 781-784). It catalyzes the reaction which gives an unstable allene oxide which can break down into the corresponding α- and γ-ketoles in the presence of water. AOS is involved in the biosynthesis of jasmonic acid (Vick and Zimmerman, 1983, Biochem. Biophys. Res. Commun. 111, 470-7). Jasmonic acid itself is involved in inducing the transcription of specific mRNA and regulating the translation of jasmonate-induced proteins (JIP), such as LOX, AOS and proteinase inhibitors. This makes jasmonic acid an important signal substance in plant stress response (Wasternack and Parthier, 1997, Trends Plant Sci. 2, 302-307). An involvement in processes of growth regulation and in the promotion of senescence is likewise described (Sembdner and Parthier, 1993, Annu. Rev. Plant Physiol. Plant Mol. Biol. 44, 569-589).
A large number of AOSs have already been cloned and expressed functionally in E. coli, including the Arabidopsis thaliana, Lycopersicon esculentum, Linum usitatissimum and Hordeum vulgare AOSs. Apart from the barley AOSs, all of the AOSs cloned to date show substrate specificity for (13S)-hydroperoxide (Maucher et al.; 2000, Plant J. 21, 199-213).
HPL (CYP74B and C) cleaves the hydroperoxide into (3Z)-aldehydes and ω-oxoacids (Matsui, 1998, Belgian Journal of Botany. 131, 50-62). Even before the enzyme itself was discovered, the HPL reaction products were known as “leaf aldehydes”, which contribute to the characteristic odor of plants and fruits (Hatanaka, 1996, Food Rev. Int. 12, 303-350). In the case of 13-HPOTE as substrate, (3Z)-hexenal and (9Z)-12-oxo-9-dodecenoic acid are formed, the latter of which isomerizes to give (10E)-12-oxo-10-dodecenoic acid (traumatin), which is discussed as a wound hormone. A function as plant messenger substance is also discussed (Bate and Rothstein, 1998, Plant J. 16, 561-569). Traumatin can be oxidized further to give traumatic acid, which likewise appears to be involved in plant wound response (Zimmerman and Vick, 1970, Plant Physiol. 46, 445-453). HPLs which were cloned and expressed in E. coli as active protein were the A. thaliana, Cucumis sativus, Medicago sativus and L. esculentum HPLs, inter alia. Again, most of these enzymes show substrate specificity for (13S)-hydroperoxide. Only one HPL from cucumber and one from melon are without substrate specificity and are therefore referred to as 9/13-HPL (McIntyre et al.; 1999, J. Biol. Chem. 274, 25189-25192; Matsui et al.; 2000, FEBS Lett. 481, 183-188). A study of the relationship of both sequences with other members of the CYP74 family reveals a higher degree of homology with AOS than with the 13-HPLs. This is why these enzymes are classified as a separate subfamily CYP74C (Matsui et al.; 2000, FEBS Lett. 481, 183-188). It has been demonstrated that the Arabidopsis HPL is induced by wounding.
DES (CYP74D) catalyzes the formation of divinyl ethers which are fungicidally active (Weber et al. 1999). An involvement of the divinyl ethers in the defense against pathogenic fungi and bacteria, analogously to the aldehydes in the case of the HPL products, is also discussed (Weber et al.; 1999, Plant Cell. 11, 485-493; Göbel et al.; 2001, J. Biol. Chem. 276, 6267-6273). The first L. esculentum DES was cloned in 2001 by Itoh and Howe. It emerged that it has a high degree of homology on a sequence with AOS and HPL; it is therefore also thought as belonging to the cytochrome P450 class, subfamily CYP74D. Moreover, DES is unique in the CYP74 group in as far as it is the only enzyme which is highly specific for 9S-hydroperoxide (Itoh and Howe, 2001, J. Biol. Chem. 276, 3620-3627).
In nature, P450 enzymes are involved in many ways in the biosynthesis and the metabolism of a large number of endogenous substances. Their involvement in the detoxification of xenobiotics is of particular importance. Moreover, plant P450 enzymes are involved in the biosynthesis of wound signals (jasmonic acid, salicylic acid, traumatin) and hormones (gibberellins, brassinosteroids).
Substances which trigger wound signals in plants and subsequent signal transduction cascades come in many forms. They can be triggered by damage or injury to the plant, but can also be induced (artificially) by external chemical compounds.
In addition, however, it is in particular the incidence of plant diseases and mainly the promotion of plant's defense reactions against such pathogens which are agronomically of enormous relevance. The plant's response to a pathogen may involve quite a different pathway than is the case when the plant defends itself against stress by wounding. The incidence of plant diseases caused by, for example, viruses, bacteria or fungi, generally results in considerable damage or indeed death of the whole plant, in conjunction with a drastic quantitative or qualitative reduction of the crop.
If the yield losses are to be limited to an economically acceptable extent, it is imperative to carry out plant protection measures. In particular, it would be desirable to improve the plants' defense reactions against pathogens and/or pests without using chemicals, which constitute an additional pollutant for the soil, the groundwater and the user.